Stabilized high-frequency amplifier



June 6, 1950 w. R. KOCH 2,510,640

STABILIZED HIGH-FREQUENCY AMPLIFIER Filed May 31, 1946 HIGH FREQUENCY AMPLIFIER I3 6 f I I r0 sou/ans 0F may omfiumrok FREQUENCY SIGNALS SIGNAL FROM POINT PRIOR 7'0 DEMODULATOR INPUT CIRCUIT I5 {I I, lz $40 Al/C 4 m SIGNAL GRIDS 5; RECTIFIER 0F CONTROLLED q AMPLIFIER TUBES 8 Li-B 4 nn; AlAlA I7 18 1: I6 air/.9

II I3 AMPLIFIER 7 7'0 OUTPUT /7 I CIRCUIT v E 1 SIG/ 4L SOURCE e INVENTOR WINFIELD R. KOCH AVC ATTORN EY Patented June 6, 1950 UNITED STATES PATENT OFFICE I STABILIZED HIGH-FREQUENCY AMPLIFIER Winfield R. Koch,'I-Iaddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 31, 1946, Serial No. 673,294

. Claims. (01. 179-171) volume control (AVC) is utilized in connection with a signal amplifier of high frequency signals, the tuned input circuit of the signal amplifier ,will actually be detuned in response to variation in magnitude of the AVG bias. Hence, it was proposed by A. Van der Zeil, U. S. Patent 2,212,851, granted August 27, 1940, to provide an unbypassed resistor in the cathode connection of the amplifier tube thereby to provide a means for minimizing the detuning of the selector input circuit of the amplifier caused by the AVG action. Such inclusion of anunbypassed cathode resistor is normally satisfactory, and provides a substantial measure of stabilization against the inherent input capacity variation caused in response to the changing of the amplifier control grid bias.

However, there are many situations encountered in high frequency apparatus wherethe use of an unbypassed cathode resistor has not been employed despite the fact that its use would have considerably aided in performance. In the case of a battery-operated radio receiver, for example, the tubes often employ filament type cathodes. In such cases it is essential that the filament of the amplifier tube be at ground potential. In the aforesaid Van der Zeil circuit the ground connection is made at the end of the cathode resistor remote from the cathode connection, and it has not been considered possible to use that circuit in connection with an amplifier employing a filament type cathode. Again, in the case of pentode tubes such as the 6SG7, where the suppressor grid is internally connected to the oathode, the grounding of the remote end of the unbypassed cathode resistor has been considered as an obstacle to use of that specific type of tube.

My present invention has as one of its main objects the provision of means for preventing changes in the selectivity curve of a high frequency signal amplifier input circuit caused by tube input capacity changes due to control grid bias variation, the amplifier circuit including a tube whose cathode is of the filament type, or .31 tube which includes a suppressor grid con- ,55 it is to bevclearly understood that the present in- 2 nected directly and internally to the cathode. In my novel and improved circuit the cathode element of the amplifier is directly connected to ground without in any way limiting the operation of the circuit with respect to the high frequency currents.

Another important object of this invention is to provide a high frequency signal amplifier whose control grid is subjected to AVC action and whose cathode is directly grounded, and which amplifier includes an unbypassed cathode resistor in a circuit common to the control grid and plate or anode circuits thereby to minimize the input capacity variation caused by the AVG action.

Still another object of this invention is to ground the cathode of a high frequency signal amplifier tube of the type including an unbypassed cathode resistor, the grounding of the cathode making it possible to use a filament type tube, or to use the type ofpentode tube whose suppressor grid is internally connected to the cathode, or to use the type of tube where it is desirable internally to connect the shell, or an internal shield, to the cathode.

Still other objects of my invention are generally to improve the usefulness and efficiency of amplifier circuits of the unbypassed cathode resistor type, and to accomplish this object with economy.

Still other features and objects of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically two circuit organizations whereby my invention may be carried into effect.

:.- In the drawing Fig. 1 shows a circuit diagram of a preferred embodiment of the invention; and

Fig. 2 shows'a modification of the controlled high frequency amplifier circuit.

Referring now to the accompanying drawing. wherein corresponding reference numerals indicate similar circuit elements throughout the figures, I have shown in Fig. 1 a high frequency signal amplifier circuit Whose input and output networks are each tuned to a predetermined high frequency carrier frequency. It is to be understood that the circuit may be included in either an amplitude modulated (AM) or frequency modulated (FM) carrier wave receiving system. While my invention is readily adapted for FM or AM reception in the presently assigned respective bands of 88-108 mo. and 550-1700 kc..

mission channels are 10 kc. wide, while the carrier wave is modulated in amplitude in accordance with the modulation signal-ls; tive radio frequency amplifier isgenerally followed by a mixer or frequency cpnverter and I.

amplifier network in cascade. Each of these nettems. Hence, I have depicted the AVG system in a schematic manner. Usually a suitable detector, or rectifier, device derives I. F. signals from any desired point prior to the demodulator input circuit, and produces a rectified control voltage. In Fig. 1 I have indicated the rectangle hi as the AVG rectifier, and it is to be clearly understood that-the connections: are made -to-any des'ired point between circuit 13 and the demodulator input circuit. The load resistor 16,

suitably bypassed for I. F. currents, has the negative end thereof connected to the low potential' end'of'ci'rcuit 4 through AVC lead l1 whichpreierably includes suitable filter resistors works is, of course, provided with suitable AM wave selector circuits. In the-case ofAMr ception, the received AM waves are usually reduced to an I. F. value of 455 kc., the latter frequency being illustrative and being-commonly employed in AM broadcast receiversof the super- The r. 51 ml amplifier network has one or mere stages of i aini lificatiori,' and in'Fig. 1- I have shown the of the I. F. signal aniiplifier circuits. The invention will be described in connection with thatparticular circuit. However, it is tdbe'clearlyunderstood that the invention equailyapplicable to any amplifier stage prior to thedemodulator ofthe receiving system. The

demodulator is not shown, since any suitable device may be employedto provide the modulation signals-which will bei'ed to a reproducer, such as a loud speaker, after suitable modulation amplification. V

The high frequency amplifier tube l, in this case-an I. F. amplifier tube, is shown as of the pentode type. Specifically, tube l is of the indirectly heated'by; a heater element. The

cathode of the tube, his shown connected directly to ground. Wherethe receiver is battery operated, thegcathocle- 2 is a filament one end of which is directly grounded. The signal input, or control; grid 3- is co'nnected towthe' high "signal" potential side of the selective input circuit 4, and the low signal potential sideof circuit 4 is connected'through'thehigh frequency bypass condenser 5 'to the lower end of the un'byp'ass'ed cathode resistor-6. The'upp'er: end

or" resistor 6 is con'nectedto the grounded cathode *2.

The resistor '6 may have a value of fro'mbi) :to 200 ohms, depending on'the tube used;

The input circuit 4 is coupled tothe resonant circuit 7 which may'bearranged 'in'the pl'ate circuit of a prior I.F. ampli'fi'er tube, or acouverter tube. The plate 8 of tube lis connected to the +13 terminal of the direct currentenergizing source (not shown) through a path consisting of primary coil 9 and resistor 19.

The-coil S is included in the resonant output circuit H which is tuned to the operating I. F. value. I. F. bypass condenser 12 connects the -ungrunded end of resistor 6 to the low potential side of output circuit 1 l. The resonant circuit 13, tuned to the LP. value,- is coupled t'o the output circuit 1 i, and feeds one or' niore -I. amplifiers followed by the demodulator. 'Condensers and i2 should havea reasonabl-ylo'w impedance at the I. F.

The use of AVG is "well known. and indeed use t8. The line "H may be connected to the signal grids of -prior 'high frequency amplifiers which are to beregulated in gain.

' If desired; there may be included a source I9 oineg'a-tive bias between ground and the positive end of resistor l6. Since cathode 2 is grounded, the source i9--will--;provide--a normal,' or nosignal, grid bias for control grid-.3. The source l9 may-be a a portion of the power supply network. -The-remaining electrodesof-tube I are connected as shown. T he screen-grid 2-8 is :conmooted-througha resistor 21 to a positive voltage point +Sof the direct "current-supply system of the receiver, the' condenseP-ZZ bypassing screen-2O to-ground. 1 Suppressor grid-23 is connected internally tothe cathode 2.

The dotted line capacity 24 denotes the inherent =control grid-to cathode'capac'ity oft'ube -l, i:=e., the input capacitance of the amplifier tube which affects the shape of the selectivity curve-when -'-the grid biasisvarie'dby AVC actiOn. It is to be clearly understood that the AVG voltage :may abe providedzby' any suitable'means responsive to icarri'er amplitude "variation. "For example, in *both' AM and FM receiversthe AVC voltage may be taken dn-eeuyrmmme demodulatorwcircuit per se.

TheiAVG circuit acts to vary the gain of each controlled amplifier tube'in a sense to compensate for Y relativelyslow carrier amplitude variation :at the signal collectordevice thereby :to' maintam th'e'carrieramplitude-at thedemodulator input circuit substantially uniform' 'over awld'e range of -'-'car'-rier amplitude variation 1 at the sig- -nal 'collctoh'flevide. e The; gri'd itocathode capacitance 24 seem-s Z w =be dependeht i in part on the "density of the electron; emud in the 's'pace between "grid Q 'and cathode-"- 2. Obviously' this density, ana -hence: the magaituue' er *capacitance: 24, changeswith hi-as enangeat grid; 3. Any change in I magnitude of oapa'cita'nc n (with resistor '6 shorted) causes a chahge the selectivity "of 4: The circuit '4 vii-111" lie-considerably The unbypassedacataeue resistor 6 puts a "coiiipb'n'ent' "of the plate enner-1t into 5 the signal grid' cii cui't seen sense-fafnd-afnp dens-" th reduce the tunihgifiect 6f tl iebapacitan'ce 24.- So far as the mam-raceway ;currents are concerire' the groufidihg f th'e cathode end-er M I then iiosite e nd doesflnot airest "the operation. 'rhereis eemepea across resistor 6 an I? F voltage which is applie'd an wide spread in the case of radio receiver sys 76; -quencyam'p1ifieis,- uae resistor 6 also Teauces'tne loading of circuit 4 by the input conductance of the tube, and makes the variations of this loading with AVG much less.

There is a decided advantage in grounding the cathode directly so far as the value of AVG bias is concerned. The AVC voltage required to control a strong signal will be less, because no direct current fiows through resistor B. The direct current of tube I flows from cathode 2 directly to the grounded side of the direct current supply source. A somewhat higher value of resistor 6 may have to be used, if the screen 20 is bypassed to ground, because screen current Will not flow through the resistor 6.

The path for radio frequency components of the plate of tube l, starting at the cathode, is through the tube (via the electron stream) to the plate, through he circuit ll, through the capacitor l2, through the resistor 6 to cathode. ground might be placed at any position in the circuit, without changing this path (except for secondary efiects). The same sort of path is followed in the grid side. The resistor 6 is the impedance common to the two circuits. No matter which end of the resistor 6 is connected to ground, the radio frequency component of the plate current will produce a voltage across this resistor, which voltage is thus introduced into the grid circuit. The radio frequency component of the screen grid current, however, does not flow through this resistor, and since the screen current is generally about of the plate current, the resistor will have to be about larger to produce the same degenerative voltage as when the screen current does flow through the resistor.

Obviously, the tube envelope can be connected internally to cathode 2 where the envelope is metallic, and the suppressor is readily internally tied to the cathode. There will, also, be less chance of radio frequency currents leaking out through the cathode to heater element capacity, where an indirectly heated cathode is employed.

In Fig. 2 I have shown an alternative embodiment of the invention. Here the cathode '2 is directly grounded, and the compensation radio frequency component is injected into the control grid circuit by means of condenser 30 and resistor 3| connected in series between the lower end of coil 9 and the low potential side of input circuit 4. This circuit is not as desirable as that of Fig. 1 since it uses an extra capacitor, while the others must be kept to a given tolerance.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

The

What I claim is:

1. In an amplifier of high frequency signals: an electron discharge tube having at least a cathode, control grid and plate; energizing means applying a, positive voltage to the plate; said cathode being grounded, a high frequency parallel resonant signal input circuit having a connection to the control grid; gain control means for applying a variable bias to the control grid; an unbypassed resistor having one end thereof connected to the grounded cathode; and respective capacitative paths from the ungrounded end of the resistor to the low signal potential sides of each of said input circuit and plate circuit.

2. In a variable-gain signal amplifier, an electron tube having at least a cathode, a control grid and an anode, said cathode being grounded, a resonant input circuit having a connection to said control grid, gain control means connected to said input circuit for applying a variable bias voltage to said control grid, a resonant output circuit having a connection to said anode, an impedance means connected between each of the low signal potential ends of said input resonant circuit and said output resonant circuit and signal ground, and a, degenerative feedback circuit connected between the junctions of said impedance means and said resonant circuits, whereby the effect of input capacitance variation caused by changes in the control grid bias voltage under the control of the automatic gain control is diminished.

3. In a variable-gain signal amplifier, the combination as defined in claim 2, wherein said degenerative feedback circuit comprises a resistor capacitor network.

4. In a variable-gain signal amplifier, the combination as defined in claim 2, wherein said degenerative feedback circuit comprises a low pass resistor-capacitor pi-section network.

5. In a variable-gain signal amplifier, the combination as defined in claim 2, wherein said degenerative feedback circuit comprises a high pass resistor-capacitor T-section network.

WINFIELD R. KOCH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,098,950 Black Nov. 16, 1937 2,173,426 Scott Sept. 19, 1939 2,179,956 Roberts Nov. 14, 1939 2,213,591 Plebanski Sept. 3, 1940 2,250,829 Foster July 29, 1941 2,262,916 Boucke Nov. 18, 1941 2,315,043 Boucke Mar. 30, 1943 2,420,249 Korm'an May 6, 1947 

